Low light vision systems, commonly referred to as night vision systems, are extremely valuable for detecting and observing distant objects at nighttime. Night vision systems are commonly incorporated in goggles which are worn by a user who looks through the goggles. A standard night vision system has a mono-focus optical lens system. Consequently, only objects that fall within the depth of focus are sharply focused. The optics of a night vision system typically have a low f number which renders a small depth of focus. Thus, the requirement for refocusing objects at different ranges is especially critical.
The need for refocusing of a monofocus night vision system is particularly noticeable when the scenery is at a far distance so that one focus is at near infinity and the other objects of interest for example a map, a dashboard, a gun sight or other piece of equipment are within a few feet of the operator. Because of the low f number of the optics, the objects at the close distance are extremely out of focus and may not even be identifiable from other out-of-focus background light.
Previous adjustment devices for changing the focus required for standard night vision systems have not been satisfactory. If the operator's situation requires his hands to be constantly in use for other pieces of equipment, a manual focusing system can be disadvantageous. Automatic focusing, on the other hand, necessitates extra hardware and extra weight associated therewith that can be cumbersome to the operator who wears the goggles. Manual and autofocusing systems are also disadvantageous for situations which require constant monitoring or attention of the distant outside scenery. Furthermore, adjustable monofocus systems whether automatic or manual are incapable of simultaneous viewing of objects at widely separated distances such as a conventional gun sight and distant target. For adequate utility, night vision system must be able to have simultaneous focusing of both the relatively close gun sight and the distant target.
One attempt at providing a dual focus night vision apparatus is disclosed in U.S. Pat. No. 4,376,889 issued to David W. Swift on Mar. 15, 1983. This system discloses a bi-focal optical system which has either an aperture or a small lens centrally located within a larger wave filter or diverging lens. The diverging lens has a wave filter incorporated therein. In another disclosed embodiment, a pair of holograms are used to focus closer objects into the night vision system. One of the holograms fills an aperture cut into a filter. The filter allows light from the distant objects to be focused into the night vision system.
There are several distinct disadvantages with the bi-focal system of U.S. Pat. No. 4,376,889. Firstly, a light vision system is extremely sensitive and its pupil needs to be adjusted to compensate for night vision during full moonlight, a new moon, or differing artificial lighting conditions. The split-lens system shown in U.S. Pat. No. 4,376,889 is incompatible with a single iris that is made to attenuate the total amount of light coming into the night vision system. More generally, diffraction problems arise with split lenses used in conjunction with a small pupil. Secondly, with the holographic lenses, the use wavelength filters are needed to provide for adequate focusing free of chromatic aberrations. The wavelength filters are used also to prevent flooding of undesired light onto the light intensifier. The wavelength filters are inefficient and require that the cockpit of an aircraft or dashboard of a motor vehicle used by the operator be lit in a particular bandwidth of light to perform correctly. The wavelength filters prevent use of these dual focus systems in vehicles that are illuminated by light outside the selected bandwidth. Thirdly, if the night vision system for selected bandwidth is inadvertently used in a vehicle having incompatible interior illumination, the operator can be placed in a dangerous situation. Furthermore, the filtering of bandwidth of light from the exterior scene further reduces available light that can be used by the light intensifier to form the intensified image under certain artificially illuminated conditions. Lastly, the formation of an aperture in a lens or filter and the placement of a second lens or hologram within the aperture requires a relatively expensive assembly process.
What is needed is an economical multi-focal night vision system that is retrofittable to standard monofocus systems. Furthermore, what is needed is a multi-focus night vision system that is compatible with an adjustable iris to attenuate the total amount of light impinging the light intensifier. What is also needed is a multi-focus night vision system that can easily proportion the amount of light focused from the near scene relative to the amount of light focused from the distant scene. A dual focus night vision system is also needed that can be generally used in any vehicle regardless of the bandwidth of the interior illumination.